1. Field of the Invention
The present invention relates to a rubber composition for adhering a steel cord that can improve adhesioness, particularly heat-resistant adhesioness and wet heat-resistant adhesioness, between a steel cord used in a reinforcing material of a pneumatic tire or the like, and a rubber.
2. Background Art
Conventionally, a plating such as brass is applied to a steel cord used in a reinforcing material of a pneumatic tire. As a rubber composition for adhesion to the steel cord, it is known that a reinforcing material such as carbon black, various compounding ingredients such as a vulcanizing agent, and additionally an adhesive component such as resorcin, a resorcin derivative or an organic acid cobalt salt are added to a diene rubber component mainly comprising a natural rubber, thereby improving adhesioness to the steel cord.
In recent years, maintenance and improvement of an expressway, and high performance of vehicles (high speed, high horsepower, high load and the like) proceed, and strain applied to a steel belt or a carcass as the framework of a pneumatic tire is markedly increased. In particular, in view of adhesion deterioration due to generation of heat in a tire during continuous running, deterioration by heat history owing to enhanced life of a tire, deterioration by wet heat, and the like, adhesioness in higher level than the conventional level is required. In the conventional rubber composition having compounded therewith an organic acid cobalt salt, initial adhesioness is excellent, but decrease in adhesioness is large due to heat aging. Thus, it is the current situation that the performance recently required cannot be satisfied, and improvement in heat-resistant adhesioness and wet heat-resistant adhesioness are strongly required.
It is considered that the organic acid cobalt salt has strong oxidation accelerating action, and therefore heat brings about to increase thickness of an adhesive layer, leading to layer failure. From this fact, JP-A-7-9811 describes that wet heat-resistant adhesioness of a steel cord is improved by containing boron in an organic acid cobalt salt. However, this still has the disadvantage that a metal cobalt component accelerates heat deterioration of a rubber composition. JP-A-2001-106829 proposes a rubber composition having added thereto a heat-resistant deterioration inhibition type vulcanization accelerator such as mercapto-4-methylbenzothiazole. However, it is difficult to say that the effect of adhesioness improvement is sufficient. WO 99/23174 A1 discloses a metal-treated carbon black having a silicon-containing phase, a boron-containing phase or the like formed on the surface of carbon black. However, this reference has an object to improve reinforcing properties of a rubber, and adhesioness is not described therein.